The invention relates to a process for the preparation of a dry aggregate for the production of electrodes.
Electrodes, for example anodes, for aluminum electrolysis, are produced from various solid and liquid carbonaceous starting materials by mixing, shaping and baking. The solid starting materials used are petroleum coke, baked and green and scrap electrode residues.
Baked electrode residues are anode residues which are recycled from the aluminum electrolysis operation and scrap from baked anodes, while green scrap occurs as waste after an electrode shaping operation and prior to the calcination or baking operation. The solid starting materials are processed in anode plants by crushing, screening or sieving and mixing to provide a granular dry substance which is subjected to further processing in mixing equipment, therein mixed with liquid binder material, generally pitch, to provide what is known as green anode material. Anodes for example are formed from the green anode material and then calcinated or baked.
In a known process for the production of a granular dry substance, crushed, baked anode residues are sieved out and stored as coarse grain in silo batteries. Petroleum coke is sieved out together with the undersize grain of the baked residues, and stored as middle grain in silos. The oversize grain which is produced in that sieving operation is crushed up and recycled while the undersize grain and the overflow material is processed in a mill to produce dust and is stored as such in a silo battery.
Coarse grain, middle grain and dust are individually withdrawn from the silos of the silo batteries by means of discharge equipment, mixed and fed to metering silos. From there the discharge is metered by way of a weighing apparatus together with granular green scrap, and mixed with pitch in kneading equipment.
In another known process, petroleum coke, baked and green anode scrap are accommodated separately in storage silos. Petroleum coke and pre-broken, baked scrap, downstream of intermediate silos, are mixed, broken and divided up by means of sieving machines into coarse, medium and fine fractions, and introduced into corresponding fraction silos. Coarse and fine mills crush the overflow material from the fraction silos for the coarse and medium grain. Undersize grain and possibly also overflow material from the fraction silos for medium and fine grain are processed in a mill to produce dust which is also passed into silos. Downstream of the fraction silos, by way of weighing equipment, the dry substance fractions, petroleum coke, baked scrap and dust are passed by way of a preheating screw and are then fed with green residues and liquid pitch pursuant to recipe provisions directly into kneading equipment.
What is common to both processes is a componentization operation, that is to say, the separate storage of the starting materials in storage silos and the dry substance components, which are made from the starting materials, in fraction silos, and gravimetric measurement of the proportions of dry substance which are intended for the mixing procedure. That componentization operation requires expensive large-scale silo equipment, both on the starting material side and on the fraction side, as well as expensive weighing equipment for each fraction.
Particular requirements in respect of quality of materials are mandatory for anodes for the aluminum electrolysis, and such requirements can be met only if the starting materials already correspond to those requirements. In the known processes, the effects of fluctuations in the levels of quality of the starting materials can only be compensated in that, in the first known process, additional silos are to be provided in the batteries of fraction silos while in the latter process, further storage silos are to be provided for starting materials of different qualities, in which case starting materials of constant average quality can be produced from the contents of the further storage silos by mixing thereof by means of additional mixing equipment.
The proportions of the starting materials for green paste are set in accordance with given compositions, which include material and geometry, that is to say grain sizes and grain size components of the starting materials. Maintaining the appropriate composition by means of the known processes is an expensive method, because of the separate preparation of the constituents of the dry material, silo storage thereof with segregation phenomena which occur as a result, and by virtue of the fraction of the green residues being brought together with the other materials, upstream of the mixing equipment, together with the gravimetric measuring operation. It has also been found that the granulometry fluctuates within wide limits around a predetermined target line.